The Full Wiki

Pleiades (star cluster): Wikis


Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.


From Wikipedia, the free encyclopedia

Pleiades large.jpg
A color-composite image of the Pleiades from the Digitized Sky Survey.
Credit: NASA/ESA/AURA/Caltech.
Observation data (J2000 epoch)
Constellation Taurus
Right ascension 3h 47m 24s[1]
Declination +24° 7′[1]
Distance 440 ly (135 pc[2][3])
Apparent magnitude (V) 1.6 [4]
Apparent dimensions (V) 110' (arcmin.) [4]
Other designations M45,[1] Seven Sisters[1]
See also: Open cluster, List of open clusters
A shorter exposure shows less nebulosity.

In astronomy, the Pleiades, or Seven Sisters (Messier object 45), is an open star cluster containing relatively young hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky. Pleiades has several meanings in different cultures and traditions.

The cluster is dominated by hot blue and extremely luminous stars that have formed within the last 100 million years. Dust that forms a faint reflection nebulosity around the brightest stars was thought at first to be left over from the formation of the cluster (hence the alternate name Maia Nebula after the star Maia), but is now known to be an unrelated dust cloud in the interstellar medium that the stars are currently passing through. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.


Names and pronunciation

See also: Pleiades in folklore and literature

The English pronunciation of the name Pleiades depends on the speaker.

  • US: /ˈpliː.ədiːz/ PLEE-ə-deez or sometimes /ˈpleɪ.ədiːz/ PLAY-ə-deez
  • UK: /ˈplaɪ.ədiːz/ PLYE-ə-deez
  • Canadian English: /ˈpleɪ.ədiːz/ PLAY-ə-deez

Other notable names of Pleiades include:

Observational history

Comet Machholz appears to pass near the Pleiades in early 2005

The Pleiades are a prominent sight in winter in the Northern Hemisphere and in summer in the Southern Hemisphere, and have been known since antiquity to cultures all around the world, including the Māori (who call them Matariki) and Australian Aborigines, the Persians (who called them Parveen/parvin and Sorayya), the Chinese, the Maya (who called them Tzab-ek), the Aztec (Tianquiztli), and the Sioux and Cherokee of North America.

The Babylonian star catalogues name them MUL.MUL or "star of stars", and they head the list of stars along the ecliptic, reflecting the fact that they were close to the point of vernal equinox around the 23rd century BC. Some Greek astronomers considered them to be a distinct constellation, and they are mentioned by Hesiod, and in Homer's Iliad and Odyssey. They are also mentioned three times in the Bible (Job 9:9 and 38:31, as well as Amos 5:8). The Pleiades (Krittika) are particularly revered in Hindu mythology as the six mothers of the war god Skanda, who developed six faces, one for each of them. Some scholars of Islam suggested that the Pleiades (Al thuraiya) are the Star in Najm which is mentioned in the Quran.

A Spitzer image of the Pleiades in infrared light, showing the associated dust. Credit: NASA/JPL-Caltech

They have long been known to be a physically related group of stars rather than any chance alignment. The Reverend John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, and so correctly surmised that the Pleiades and many other clusters of stars must be physically related.[5] When studies were first made of the stars' proper motions, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related.

Charles Messier measured the position of the cluster and included it as M45 in his catalogue of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe cluster, Messier's inclusion of the Pleiades has been noted as curious, as most of Messier's objects were much fainter and more easily confused with comets—something which seems scarcely possible for the Pleiades. One possibility is that Messier simply wanted to have a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, and so he added some bright, well-known objects to boost his list.[6]


The distance to the Pleiades is an important first step in the so-called cosmic distance ladder, a sequence of distance scales for the whole universe. The size of this first step calibrates the whole ladder, and the scale of this first step has been estimated by many methods. As the cluster is so close to the Earth, its distance is relatively easy to measure. Accurate knowledge of the distance allows astronomers to plot a Hertzsprung-Russell diagram for the cluster which, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods can then extend the distance scale from open clusters to galaxies and clusters of galaxies, and a cosmic distance ladder can be constructed. Ultimately astronomers' understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades.

Results prior to the launch of the Hipparcos satellite generally found that the Pleiades were about 135 parsecs away from Earth. Hipparcos caused consternation among astronomers by finding a distance of only 118 parsecs by measuring the parallax of stars in the cluster—a technique which should yield the most direct and accurate results. Later work has consistently found that the Hipparcos distance measurement for the Pleiades was in error, but it is not yet known why the error occurred.[7] The distance to the Pleiades is currently thought to be the higher value of about 135 parsecs (roughly 440 light years).[2][3][8]


X-ray images of the Pleiades reveal the stars with the hottest atmospheres. Green squares indicate the seven optically brightest stars.

The cluster core radius is about 8 light years and tidal radius is about 43 light years. The cluster contains over 1,000 statistically confirmed members, although this figure excludes unresolved binary stars.[9] It is dominated by young, hot blue stars, up to 14 of which can be seen with the naked eye depending on local observing conditions. The arrangement of the brightest stars is somewhat similar to Ursa Major and Ursa Minor. The total mass contained in the cluster is estimated to be about 800 solar masses.[9]

The cluster contains many brown dwarfs, which are objects with less than about 8% of the Sun's mass, not heavy enough for nuclear fusion reactions to start in their cores and become proper stars. They may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of the total mass.[10] Astronomers have made great efforts to find and analyse brown dwarfs in the Pleiades and other young clusters, because they are still relatively bright and observable, while brown dwarfs in older clusters have faded and are much more difficult to study.

Age and future evolution

Ages for star clusters can be estimated by comparing the Hertzsprung-Russell diagram for the cluster with theoretical models of stellar evolution, and using this technique, ages for the Pleiades of between 75 and 150 million years have been estimated. The spread in estimated ages is a result of uncertainties in stellar evolution models. In particular, models including a phenomenon known as convective overshoot, in which a convective zone within a star penetrates an otherwise non-convective zone, result in higher apparent ages.

Another way of estimating the age of the cluster is by looking at the lowest-mass objects. In normal main sequence stars, lithium is rapidly destroyed in nuclear fusion reactions, but brown dwarfs can retain their lithium. Due to lithium's very low ignition temperature of 2.5 million kelvins, the highest-mass brown dwarfs will burn it eventually, and so determining the highest mass of brown dwarfs still containing lithium in the cluster can give an idea of its age. Applying this technique to the Pleiades gives an age of about 115 million years.[11][12]

The cluster's relative motion will eventually lead it to be located, as seen from Earth many millennia in the future, passing below the feet of what is currently the constellation of Orion. Also, like most open clusters, the Pleiades will not stay gravitationally bound forever, as some component stars will be ejected after close encounters and others will be stripped by tidal gravitational fields. Calculations suggest that the cluster will take about 250 million years to disperse, with gravitational interactions with giant molecular clouds and the spiral arms of our galaxy also hastening its demise.

Reflection nebulosity

Hubble Space Telescope image of reflection nebulosity near Merope

Under ideal observing conditions, some hint of nebulosity may be seen around the cluster, and this shows up in long-exposure photographs. It is a reflection nebula, caused by dust reflecting the blue light of the hot, young stars.

It was formerly thought that the dust was left over from the formation of the cluster, but at the age of about 100 million years generally accepted for the cluster, almost all the dust originally present would have been dispersed by radiation pressure. Instead, it seems that the cluster is simply passing through a particularly dusty region of the interstellar medium.

Studies show that the dust responsible for the nebulosity is not uniformly distributed, but is concentrated mainly in two layers along the line of sight to the cluster. These layers may have been formed by deceleration due to radiation pressure as the dust has moved towards the stars.[13]

Brightest stars in Pleiades

A map of the Pleiades

The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygete, Celaeno, and Alcyone, along with their parents Atlas and Pleione. As daughters of Atlas, the Hyades were sisters of the Pleiades. The English name of the cluster itself is of Greek origin, though of uncertain etymology. Suggested derivations include: from πλεîν plein, to sail, making the Pleiades the "sailing ones"; from pleos, full or many; or from peleiades, flock of doves. The following table gives details of the brightest stars in the cluster:

Pleiades Bright Stars
Name Pronunciation (IPA & respelling) Designation Apparent magnitude Stellar classification
Alcyone /ælˈsaɪ.əniː/ al-SYE-ə-nee Eta (25) Tauri 2.86 B7IIIe
Atlas /ˈætləs/ AT-ləs 27 Tauri 3.62 B8III
Electra /ɪˈlɛktrə/ i-LEK-trə 17 Tauri 3.70 B6IIIe
Maia /ˈmeɪə, ˈmaɪə/ MAY, MYE 20 Tauri 3.86 B7III
Merope /ˈmɛrəpiː/ MERR-ə-pee 23 Tauri 4.17 B6IVev
Taygeta /teɪˈɪdʒɪtə/ tay-IJ-i-tə 19 Tauri 4.29 B6V
Pleione /ˈplaɪ.əniː/ PLYE-ə-nee 28 (BU) Tauri 5.09 (var.) B8IVep
Celaeno /sɪˈliːnoʊ/ sə-LEE-noh 16 Tauri 5.44 B7IV
Sterope, Asterope /ˈstɛrɵpiː, əˈstɛrɵpiː/ (ə)-STERR-o-pee 21 and 22 Tauri 5.64;6.41 B8Ve/B9V
18 Tauri 5.65 B8V

Cultural significance

Japanese automaker Subaru derives its name from the cluster, which is represented in its corporate logo.[14] See the article Pleiades in folklore and literature.

See also


  1. ^ a b c d "SIMBAD Astronomical Database". Results for M45. Retrieved 2007-04-20.  
  2. ^ a b Percival, S. M.; Salaris, M.; Groenewegen, M. A. T. (2005), The distance to the Pleiades. Main sequence fitting in the near infrared, Astronomy and Astrophysics, v.429, p.887.
  3. ^ a b Zwahlen, N.; North, P.; Debernardi, Y.; Eyer, L.; Galland, F.; Groenewegen, M. A. T.; Hummel, C. A. (2004), A purely geometric distance to the binary star Atlas, a member of the Pleiades, Astronomy and Astrophysics, v.425, p.L45.
  4. ^ a b Messier 45
  5. ^ Michell J. (1767), An Inquiry into the probable Parallax, and Magnitude, of the Fixed Stars, from the Quantity of Light which they afford us, and the particular Circumstances of their Situation, Philosophical Transactions, v. 57, p. 234-264
  6. ^ Frommert, Hartmut (1998) "Messier Questions & Answers". Retrieved March 1, 2005.
  7. ^ Soderblom D.R., Nelan E., Benedict G.F., McArthur B., Ramirez I., Spiesman W., Jones B.F. (2005), Confirmation of Errors in Hipparcos Parallaxes from Hubble Space Telescope Fine Guidance Sensor Astrometry of the Pleiades, The Astronomical Journal, v. 129, pp. 1616-1624.
  8. ^ Turner, D. G. (1979),[1], Publications of the Astronomical Society of the Pacific, v. 91, pp. 642-647.
  9. ^ a b Adams, Joseph D.; Stauffer, John R.; Monet, David G.; Skrutskie, Michael F.; Beichman, Charles A. (2001), The Mass and Structure of the Pleiades Star Cluster from 2MASS, The Astronomical Journal, v.121, p.2053.
  10. ^ Moraux, E.; Bouvier, J.; Stauffer, J. R.; Cuillandre, J.-C. (2003), [ Brown in the Pleiades cluster: Clues to the substellar mass function], Astronomy and Astrophysics, v.400, p.891.
  11. ^ Basri G., Marcy G. W., Graham J. R. (1996), Lithium in Brown Dwarf Candidates: The Mass and Age of the Faintest Pleiades Stars, Astrophysical Journal v.458, p.600
  12. ^ Ushomirsky, G., Matzner, C., Brown, E., Bildsten, L., Hilliard, V., Schroeder, P. (1998), Light-Element Depletion in Contracting Brown Dwarfs and Pre-Main-Sequence Stars, Astrophysical Journal v.497, p.253
  13. ^ Gibson, Steven J.; Nordsieck, Kenneth H. (2003), The Pleiades Reflection Nebula. II. Simple Model Constraints on Dust Properties and Scattering Geometry, The Astrophysical Journal, v.589, p. 362
  14. ^

External links

Coordinates: Sky map 03h 47m 24s, +24° 07′ 00″


Simple English

[[File:|thumb|Pleiades star cluster]] The Pleiades (also known as M45 or the Seven Sisters) are a group of stars in the night sky. You can see them in the constellation (pattern of stars) called Taurus, the bull. An astronomer named Charles Messier gave it the name M45. They are named after the Pleiades in Greek mythology.

Error creating thumbnail: sh: convert: command not found
Wikimedia Commons has images, video, and/or sound related to:


Got something to say? Make a comment.
Your name
Your email address